A nuclear medicine diagnostic method images radio-pharmaceutical distributions using a scintillation camera. A radio-pharmaceutical, a compound containing a gamma ray-emitting redionuclide, is administered to a patient and concentrates in tissues to be diagnosed. A scintillation camera is used to subsequently detect gamma ray emission to analyze the trajectory so that one or two-dimensional projected images of the radionuclide distributions may be constructed. The scintillation camera may be the type disclosed in U.S. Pat. No. 3,011,057. In this type of patent, the detector is a planar sodium-iodide scintillation crystal coupled through one or more light pipes to a one or two-dimensional array of closely packed photomultipliers. The light from the scintillation event is distributed to the photomultipliers both directly and as a result of reflection within the scintillator and light pipe. The scintillation crystal converts the gamma rays to light, to which the photomultipliers are responsive. The photomultipliers convert the collected light into an electronic pulse, the charge of which is proportional to the quantity of light received and thus, too, to the energy of the gamma ray received.
Recently, cameras having arcuate, one-dimensional, and cylindrical, two-dimensional, geometries, have been suggested by the applicants herein, Genna and Pang, in the parent application. Such cameras are particularly well suited for use in the reconstruction of transaxial emission images from gamma ray projections, using emission computerized axial tomography.
The determination of the position of a scintillation event, position analysis, from the partition of light to photomultipliers, presents formidable non-linearity and non-isotropicity problems which are not satisfactorily solved by known analog techniques. These problems generally manifest themselves as degraded energy and position resolution, image distortions, and non-uniformity. The problems are compounded when the camera uses unusual detector geometries, such as arcuate and cylindrical. The analog methods are generally based on a linear analysis of a system in which the partition of light is non-linear. That is, the response of a photomultiplier or a group of photomultipliers in a scintillation camera is not a linear function of the displacement of the scintillation event from the photomultiplier axes. Non-isotropicity derives from the inherent non-isotropic crystal and photomultiplier response, non-uniform crystal and light pipe boundary reflectivity conditions, non-uniform optical couplings, and end effects. Prior art techniques attempt to improve linearity by manipulating the light collection geometry and the photomultiplier signals prior to the position analysis. For example, in the above identified U.S. patent the planar photomultiplier surface is a substantial distance from sources of scintillation in order to improve linearity. In U.S. Pat. No. 3,919,556, attempts to improve linearity include use of spherically shaped photomultiplier windows. Other well-known techinques include light pipe shaping; light masking of the interfaces between the light pipe and the photomultiplier windows; selective light absorption at external surfaces such as crystal edges and light pipe surfaces between photomultiplier windows; and non-linear electronics, such as threshold amplifiers. However, these techniques exchange improved linearity for diminished and poorer photon statistics, which ultimately manifest as degradation of energy and position resolution, as well as non-uniformity. Typically the uniformity of prior art in analog position analysis devices varies by as much as plus or minus 15% over the aperture of the collimator.